1. Field of the Invention
The present invention relates to a noise masking device and a noise masking method of an image forming apparatus such as an office automation equipment, a laser beam printer and an electrophotographic copier, using a drive motor which becomes a source of a noise upon operation thereof, and more particularly, to a noise masking device and a noise masking method for generating a noise masking sound which auditorily drowns out the noise produced by the drive motor of the noise source, to thereby drown out the noise which causes unpleasant feeling.
2. Description of the Related Art
Conventionally, a plurality of mechanical drive motors are utilized as drive motors for an image forming apparatus such as a laser beam printer and an electrophotographic copier. The recent digitization brings use of a special drive motor.
For example, an image carrier (document) is scanned by a fluorescent tube or an LED to read the image by a CCD in a digitized image forming apparatus. Recording of the image (making a document) is performed with an image recording apparatus by scanning a recording medium with a light beam from a light source such as a laser diode which is modulated with an image signal or a character signal. In this case, a light deflector which includes a polygon mirror having a plurality of reflecting facets on its outer peripheral and a drive motor for rotating the polygon mirror is used as a optical scanning device for scanning the light beam. An example of the drive motor of such light deflector will be subsequently explained.
FIG. 12 is a perspective view explaining a construction of a light deflector (an optical scanning device). In FIG. 12, designated at numeral 11 is a drive motor, 12 is a polygon mirror, 13 is a laser source, 14 is a collimator lens, 15 is a converging optical member (a converging lens) and 16 is a recording member (a photosensitive drum).
The construction of the light deflector in the image forming apparatus and the outline of the image forming method is schematically explained with reference to FIG. 12. On performing the image recording, the polygon mirror 12 is rotated by the drive motor 11 in the direction shown by an arrow A. The laser source 13 is constituted by a semiconductor laser or a gas laser. The light beam emitted from the laser source 13 is modulated with an image signal by a modulator not shown and is incident on one of the reflecting facets of the polygon mirror 12 through the collimator lens 14. The light beam reflected by the reflecting facet of the polygon mirror 12 is projected to the recording member 16 through the converging optical member 15. In this case, the reflected light beam is deflected in the direction shown by an arrow B for main-scanning the recording member 16 in accordance with the rotation of the polygon mirror 12 in the direction of the arrow A. With this main-scanning, the recording member 16 is rotated in the direction shown by an arrow C to perform sub-scanning, resulting in that a two-dimensional image is written on the recording member 16.
The drive motor used in such light deflector, for example, includes a rotational bearing such as a dynamic pressure air bearing or a ball bearing having a sleeve and a shaft mutually engaging, one of which is a rotational member and the other is a fixed member, a permanent magnet mounted to the rotational member and a magnetic circuit having an electromagnetic coil wound around a circular iron core mounted to the fixed member, whereby a rotational torque is generated. Namely, the drive motor has the magnetic circuit serving as a magnetic bearing for supporting the rotational member in an axial direction.
Therefore, a noise occurs due to the operation of the drive motor 11 when the above-mentioned image recording is performed. The noise produced according to the variation in the rotational speed of the drive motor is explained. As shown in FIG. 13, the noise is produced and varied in accordance with the rotational speed of the drive motor of the light deflector. FIG. 13 shows a process from when the power source of the image forming apparatus is turned on to when a series of image forming is terminated as a variation in the rotational speed (the noise) of the drive motor.
As shown in FIG. 13, when the power source is turned on, the rotational speed of the drive motor increases to a predetermined speed. If a predetermined operation does not start after keeping a constant rotational speed, the apparatus enters into a stand-by mode and decreases the rotational speed to enter into a resting state. Thereafter, when a beginning of procedures is instructed, a speed-rising of the drive motor is started. When the rotational speed of the drive motor increases to the predetermined speed, an operational drive of the drive motor is started for the predetermined procedures. After the operational drive of the drive motor is terminated, a part of fans stops rotating. The rotation in the predetermined period for cooling is continued and then the deceleration of the drive motor is started to decrease the rotational speed to that in the stand-by mode. Thereafter, the rotational speed of the stand-by mode is kept to maintain the stand-by state.
Namely, in the case where the procedures are not performed for a short while after the power source is turned on, the image forming apparatus is brought into the stand-by mode after a several to several ten seconds from the turning-on of the power source to restrain the consumption of electricity during the stand-by mode, so that almost all the mechanisms except for a radiation fan become in the resting state. Ordinarily, preparatory to the next operational drive, the light deflector during the stand-by mode is decelerated and driven at almost a half of the predetermined rotational speed in order to shorten a speed-rise period from when the drive motor starts up to when reaches the predetermined rotational speed. In recent years, there has been provided a type wherein the rotational speed falls down to zero in the stand-by mode in order to further reduce the consumption of the electricity during the stand-by mode.
When an operator pushes a button on a control panel to input a process start signal during the stand-by mode for performing the image forming process, the image forming apparatus is brought into an operational mode, whereby the drive motor of the light deflector is starting up to accelerate until reaching the predetermined rotational speed. At this time, it is necessary to rotate the drive motor of the light deflector at high speed in a short period from the viewpoint of an image forming cycle. Therefore, the drive motor of the light deflector is constructed to be used with the rotational speed higher than that of the general motor, so that the rotational speed of the drive motor of the light deflector is 5,000 rotations or more, and 10,000 rotations or more per minute according to the situation. Accordingly, a large electricity is sent to the drive motor upon starting-up to rapidly increase the rotational speed, to thereby produce a large noise. This noise is a fluctuating sound following the fluctuation of the rotational speed, and gets greatly on persons nerve to bring unpleasant feeling.
The image forming process is started to perform a series of procedures after the drive motor reaches the predetermined rotational speed. After the completion of the procedures, each mechanism of the apparatus enters into the resting state. In the case where the next procedure is not performed after a predetermined period, the image forming apparatus is changed over to the stand-by mode, and the drive motor enters a decelerated drive.
Conventionally, there has been proposed a technique for reducing the jarring noise occurring due to the frequency fluctuation. For example, Japanese Unexamined Patent Publication Nos. Sho 63-59797 (1988) and Hei 6-175443 (1994) propose such a technique. The former proposes a driving method for a stepping motor wherein a time variation of a frequency upon the speed-rise time of the drive motor is constructed to form a plurality of curve lines to ease a rapid change. Further, the latter proposes an image forming apparatus in which other operations of the image forming apparatus are separately and precedentially performed upon the speed-rise time of a driver motor for a polygon mirror so that the operational sounds overlap the sound of the drive motor for masking.
According to the above-mentioned conventional example, the time variation of the frequency is constructed to form a plurality of curve lines to eliminate psychologically unpleasant feeling due to the noise produced upon the speed-rise time of the drive motor of the light deflector, thereby being effective to some degree in easing the rapid change in the sound. However, the frequency fluctuation is almost recognized, so that it does not come to eliminate unpleasant feeling.
In the method according to the above-mentioned other conventional example in which the operational sound overlaps with the noise of the drive motor to mask the noise, the noise (sound volume) of the whole sound further increases to become noisy. Therefore, it is difficult to eliminate unpleasantness. During the period from when the power source is turned on or from when the image forming process is terminated to when the apparatus is changed over to the stand-by mode, the other portions of the image forming apparatus is continuously and separately operated, resulting in that it is unfavorable from the viewpoint of the reduction in the consumption of the electricity.